1. Field of the Invention
The invention relates generally to automatic clothes washers, and, more specifically relates to an automatic clothes washer and method for determining an unbalanced condition, especially a dynamic unbalanced condition.
2. Description of the Related Art
Washing machines utilize a generally cylindrical perforated basket for holding clothing and other articles to be washed that is rotatably mounted within an imperforate tub mounted for containing the wash liquid, which generally comprises water, detergent or soap, and perhaps other constituents. In some machines the basket rotates independently of the tub and in other machines the basket and tub both rotate. Typically, an electric motor drives the basket. Various wash cycles introduce into the clothing and extract from the clothing the wash liquid, usually ending with one or more spin cycles where final rinse water is extracted from the clothes by spinning the basket.
It is common to categorize washing machines by the orientation of the basket. Vertical-axis washing machines have the basket situated to spin about a vertical axis relative to gravity. Horizontal-axis washing machines have the basket oriented to spin about an essentially horizontal axis relative to gravity.
Both vertical and horizontal-axis washing machines extract water from clothes by spinning the basket about their respective axes, such that centrifugal force extracts water from the clothes. Spin speeds are typically high in order to extract the maximum amount of water from the clothes in the shortest possible time, thus saving time and energy. But when clothing and water are not evenly distributed about the axis of the basket, an imbalance condition occurs. Typical spin speeds in a vertical axis washer are 700-800 RPM, and in a horizontal axis washer at 1000-1200 RPM. At such high speeds, an imbalance can result in unacceptable vibratory movement of the basket and the entire washing machine. The washing machine can be affected severely enough that it will “walk” across the floor and cause floor vibration. The tub and basket can move enough such that the tub reaches the limit of its suspension and/or contacts the surrounding cabinet structure, referred to as “cabinet hits,” with consequent noise and possible damage.
Moreover, demand for greater load capacity fuels a demand for larger baskets. Higher spin speeds coupled with larger capacity baskets aggravates imbalance problems in washing machines, especially in horizontal axis washers. Imbalance conditions become harder to accurately detect and correct.
As the washing machine basket spins about its axis, there are generally two types of imbalances that it may exhibit: static (single) imbalance and dynamic (coupled) imbalance. FIGS. 1-4 illustrate schematically different configurations of imbalance in a horizontal axis washer 10 having a perforate basket 18 having a horizontal geometric axis 21, and coaxially enclosed within an imperforate, stationary tub 20 having a front 15 with an opening 30 (through which access to the interior of the basket 18 is normally provided) and a back 17. The tub 20 is suspended by one or more springs 32 within a cabinet 12. A drive point 19 (usually a motor shaft) is typically located at the back 17. One or more dampers or shock absorbers 34 are attached to the tub 20, generally diametrically opposite the springs 32.
FIGS. 1A and B show a single or static imbalance condition generated by a single off-balance load 80. Imagine a load 80 on one side of the basket 18, but centered between the front 15 and the back 17. A torque t caused by the magnitude of the imbalance is equal tot=mgR                Where m=mass of the imbalance;        g=gravitational acceleration;        R=radial location of the imbalance.        The suspension system having the springs 32 and the dampers 34 is designed to handle such vibration under normal conditions. During rotation the motor will consume energy to lift the imbalance weight, or overcome the torque t. Therefore static imbalances are detectable at relatively slow speeds such as 85 or 100 RPM by measuring the fluctuation of speed, current, or watts of the driving motor.        
Coupled or dynamic imbalance is shown in FIG. 2. Imagine a dynamic off balance load of two identical masses 82, one on one side of the basket 18 near the front 15 and the other near the back 17. In other words, the masses 82 are on a line 84 skewed relative to the geometric axis 21. The torque t due to imbalance gravity about the geometric axis 21 is zero, so there is no fluctuation of speed, current or watts and the motor cannot detect current the imbalance. However, there is a net moment torque M, so that the basket 18 will tend to wobble about an axis perpendicular to the plane of FIG. 2B. If the moment is high enough, the wobble can be unacceptable.
FIGS. 3A and B illustrate a single imbalance caused by a front off-balance load. Imagine a single load 86 in the basket 18 toward the front 15. There is a torque t due to imbalance gravity about the geometric axis 21. There is also a moment M, about an axis perpendicular to the plane of FIG. 3B.
FIGS. 4A and B illustrate a single imbalance caused by a rear off-balance load. Imagine a single load 88 in the basket 18 toward the back 17. There is a torque t due to imbalance gravity about the geometric axis 21. There is also a moment M, about an axis perpendicular to the plane of FIG. 4B.
A single imbalance load is detectable above a certain speed at which the clothes load settles inside the basket. At the static imbalance detection speed (about 85-100 RPM for a horizontal axis washer), the torque t is transferred to the motor shaft, causing speed or power fluctuation in the motor. But the estimated value is related only to the effect of the static imbalance. For instance, in FIGS. 1, 3 and 4, the three single imbalance loads yield an identical value regardless of whether the load is located at the front as in FIG. 3 or the back as in FIG. 4. This single static imbalance is correlated to the magnitude of the imbalance. However, dynamically, there is a significant difference when an imbalance load is in the front or at the back. The front imbalance load in FIG. 3 has a much larger moment M compared with that of the back imbalance load in FIG. 4, because the instant pivot point is at the rear bearings support area. For simplicity, we will assume pivot point is at the front bearing for later discussion.
The coupled dynamic imbalance effect in a horizontal axis washing machine can be seen in FIG. 5, where the magnitude of the imbalance load, in kilograms, and the dynamic moment (or location of the imbalance back to front) are defined as two axes in a Cartesian coordinate plane. In this plane, the whole area is separated into two parts by a dynamic moment limit curve BE defined by the tolerances of the particular washing machine. BE represents the acceptable moment with respected to vibration level that is related to the effects of dynamic imbalance load at a given RPM. There are a set of such curves corresponding to different high spinning speeds. The area above this limit curve is the unacceptable imbalance area at a given spinning speed. The area below the limit curve is the accepted operating region. Note, as explained above, that there is a significant difference in the effect of the moment on the curve BE between the front and the back. The imbalance at the front has larger dynamic effects that result in larger vibration.
Imagine detecting only a single static imbalance using current motor speed, current, or watts technology. To avoid severe vibration at the front, a low limit setting (at line AB) must be established in the washing machine by assuming a worst case. Consequently, all area between the curve BE and above the line AB represents an overestimated difference between the actual speed permitted by the motor controller (limited by line AB) and the maximum speed at which the machine could operate (limited by the curve BE. If the limit setting is established higher, as at the line CD, the area between the curve BE and below the line CD represents an underestimate for a front imbalance, and the area between the curve BE and above the line CD represents an overestimate for a rear imbalance. A consequent result is unacceptable vibration and noise at high speed due to the underestimate. Thus, there is an additional need to detect the location of an imbalance load in a horizontal axis washing machine, as well as the existence of any coupled dynamic imbalance.
Many efforts have been put for detecting location of single static imbalance, as well as coupled dynamic imbalance but not successful. Many solutions have been advanced for detecting and correcting single static imbalance but correction is generally limited to aborting the spin, reducing the spin speed, or changing the loads in or on the basket. Detection presents the more difficult problem. It is known to detect vibration directly by employing switches, such as mercury or micro-switches, which are engaged when excessive vibrations are encountered. Activation of these switches is relayed to a controller for altering the operational state of the machine. It is also known to use electrical signals from load cells on the bearing mounts of the basket, which are sent to the controller. Other known methods sample speed variations during the spin cycle and relate it to power consumption. For example, it is known to have a controller send a PWM (Pulse Width Modulated) signal to the motor controller for the basket, and measure a feedback signal for RPM (Rotations Per Minute) achieved at each revolution of the basket. Fluctuations in the PWM signal correspond to basket imbalance, at any given RPM. Yet other methods measure power or torque fluctuations by sensing current changes in the drive motor. Solutions for detecting static imbalances by measuring torque fluctuations in the motor abound. But there is no correlation between static imbalance conditions and dynamic imbalance conditions; applying a static imbalance algorithm to torque fluctuations will not accurately detect a dynamic imbalance.
For example, an imbalance condition caused by a front off balance load (see FIG. 3) will be underestimated by existing systems for measuring static imbalances. Conversely, an imbalance condition caused by a rear off balance load (see FIG. 4) will be overestimated by existing systems for measuring static imbalances.
Moreover, speed, torque, current, watts in the motor can all fluctuate for reasons unrelated to basket imbalance. For example, friction conditions can change over time and from system to system. Friction in a washing machine has two sources. One may be called “system friction.” Because of differences in the bearings, suspension stiffness, machine age, normal wear, motor temperature, belt tension, and the like, the variation of system friction can be significantly large between one washing machine and another. A second source of friction in a given washing machine is related to load size and any imbalance condition. Commonly owned U.S. Pat. No. 6,640,372 presents a solution to factoring out conditions unrelated to basket imbalance by establishing a stepped speed profile in which average motor current is measured at each step and an algorithm is applied to predetermined thresholds for ascertaining an unbalanced state of the basket. Corrective action by the controller will reduce spin speed to minimize vibration. The particular algorithm in the '372 patent may be accurate for ascertaining static imbalances. However, it is not entirely accurate for horizontal axis washing machines because it does not accurately ascertain the various dynamic imbalance conditions and does not ascertain information related to load size.
Another problem in reliably detecting imbalances in production washers regardless of axis is presented by the fact that motors, controllers, and signal noise vary considerably from unit to unit. Thus, for example, a change in motor torque in one unit may be an accurate correlation to a given imbalance condition in that unit, but the same change in torque in another unit may not be an accurate correlation for the same imbalance condition. In fact, the problems of variance among units and signal noise are common to any appliance where power measurements are based on signals that are taken from electronic components and processed for further use.
Prior art horizontal axis washing machines utilize motor torque, or current, motor speed, and motor watts, to detect a load imbalance. However such technologies cannot detect coupled dynamic loads, are unable to base corrective action on the location of the imbalance, overcompensate for load imbalances at the rear of the basket and under compensate for load imbalances at the front of the basket. Accelerometers are utilized to monitor vibration and enable preventative measures to be taken to avoid catastrophic vibration at high speeds, i.e. 400 RPM and above. However the critical speed for vibration-caused cabinet hits is typically between 160 and 200 RPM, well before the 400 RPM speed is reached. Furthermore, the use of accelerometers typically does not enable the determination of the imbalance location, or the severity of vibration at higher speeds. However, accelerometers have the advantage of low cost, a well-understood operational theory, and performance unaffected by system friction, or variation in motors, controllers, and signal noise.
There exists a need in the art for an accelerometer-based imbalance detection system for a washing machine, particularly horizontal axis washing machines, which can effectively, efficiently, reliably and accurately sense load size, the existence and magnitude of any imbalance condition, and sense other obstructions that may adversely affect performance. Further, there is a need for accurately determining stable and robust power information that can accommodate variations in motors, controllers, system friction, and signal noise from unit to unit.